It has long been desirable to engineer metallic foam materials. Metallic foam structures (metallic solid foam or metallic cellular solids) are known to have interesting combinations of physical properties. Metallic foams offer high stiffness in combination with very low specific weight, high gas permeability, and a high energy absorption capability. As a result, these metallic foam materials are emerging as a new engineering material. However, metals in their crystalline state exhibit poor plasticity, and the plastic deformation that can be accomplished during the production of these crystalline metallic foams is also limited. Researchers have attempted to overcome the problems of mechanical failures due to dislocations and grain boundaries, and to produce desirable magnetic and electrical properties by producing foams out of metallic glasses through the rapid quenching of thin streams of amorphous material.
Indeed, it has been about 30 years since Paul Duwez and colleagues demonstrated that metallic glasses from the melt can be produced using his “gun technique” if the quench rate was sufficiently rapid (e.g., .about.106 K/S) (See P. Duwez, R. H. Willens, and W. Klement, Jr., J. Appl. Phys. 31, 135 (1960)). Since that time much experimental and theoretical work has disclosed the conditions necessary to produce and maintain the metallic glass (“amorphous”) state. David Turnbull has been among the leaders in the field. His work in the late 40's with metal alloy (mercury) drops and that of Vonnegut with oxide coated tin drops demonstrated that the undercooling of metallic materials followed a path similar to non-metallic materials. (See, e.g., D. Trumbull, J. Appl. Phys. 20, 817 (1949) and B. Vonnegut, J. Colloid Sci. 3, 563 (1948)). Deep undercoolings were possible if heterophase nucleants were either absent or neutralized. Even relatively large samples (e.g. a few grams) could be undercooled if nucleants were removed by appropriate fluxing techniques.
However, in order for a glass to form, the melt must reach the glass forming temperature, Tg, before crystal nucleation can occur. The material must thus undercool below the liquidus temperature, T1, in order to reach Tg. The reduced glass temperature ratio Trg=Tg/T1 becomes an important parameter. For example, when produced by solidifying the molten alloy into a solid state, the structure of these alloys becomes to be amorphous only at high cooling rates of around 1-250 K/s. Moreover, even with these high cooling rates, the maximum size attainable by this method is around 10 mm in diameter.
In light of these manufacturing constraints, the production of metallic foamed structures is generally carried out in the liquid state above the melting temperature of the material. The foaming of ordinary metals is challenging because a foam is an inherently unstable structure. The reason for the imperfect properties of conventional metallic foams comes from the manufacturing process itself. For example, although a pure metal or metal alloy can be manufactured to have a large volume fraction (>50%) of gas bubbles, a desired bubble distribution cannot be readily sustained for practical times while these alloys are in their molten state. This limitation also results in difficulties in attempts to produce continuously cast parts with different thicknesses and dimensions. Specifically, the time scales for the flotation of bubbles in a foam scales with the viscosity of the material. Most conventional alloys have a very low viscosity in the molten state. Accordingly, the mechanical properties of these foams are degraded with the degree of imperfection caused by the flotation and bursting of bubbles during manufacture. In addition, the low viscosity of commonly used liquid metals results in a short time scale for processing, which makes the processing of metallic foam a delicate process.
In order to remedy these shortcomings, several techniques have been attempted. For example, to reduce the sedimentation flotation process, Ca particles may be added to the liquid alloy. However, the addition of Ca itself degrades the metallic nature of the base metal as well as the resultant metallic foam. Alternatively, foaming experiments have been performed under reduced gravity, such as in space, to reduce the driving force for flotation; however, the cost for manufacturing metallic foams in space is prohibitive.
One promising technique would appear to be powder consolidation. U.S. Pat. No. 3,087,807, the disclosure of which is incorporated herein by reference, teaches a method which permits the manufacture of a porous metal body of any desired shape. According to this method, a mixture of a metal powder and a propellant powder is cold-compacted at a compressive pressure of at least 80 MPa in a first step. Subsequent extrusion molding reshapes it at least 87.5%. This high degree of conversion is necessary for the friction of the particles with one another during the shaping process to destroy the oxide coatings and bond the metal particles together. The extruded rod thus produced can be foamed to form a porous metal body by heating it at least to the melting point of the metal. Foaming can be performed in various molds so that the finished porous metal body has the desired shape.
However, no one has successfully made an amorphous metallic foam through consolidation with an amorphous metallic powder. The reason for this is that many current powder consolidation techniques require the use of very high temperatures for compacting and bonding of the metal particles, and the temperature cannot be set at an arbitrary level, as is often required to maintain the amorphous qualities of the amorphous powders.
For example, U.S. Pat. No. 4,523,621 discloses a method for making amorphous powder and consolidating this powder by a hot extrusion. In this example, powders are made by a gas atomization method under the rapid solidification condition. Amorphous powder selected from them is contained in a Cu container and sealed. Then, the amorphous powder is consolidated beyond the amorphous transition temperature by a hot extrusion or a hot forging to obtain a bulk amorphous material without size limitation.
However, in the method described in the '621 patent, it is often difficult to consolidate the powder under the condition of maintaining the vitreous state. That is, in order to prevent crystallization in the amorphous alloy, extrusion ratio needs to be reduced. Furthermore, an oxide layer generally formed on the surface of the amorphous powders can reduce the bonding strength between the amorphous powders. Due to the disadvantages mentioned above, the product contains micro-voids between the particles. In addition, in order to prevent the formation of the oxide layer, the entire fabrication processes should be carried out under an Ar gas or vacuum condition, thereby increasing the production cost. Further, after extrusion, the produced sample must be rapidly cooled to prevent crystallization.
Accordingly, a need exists for an improved method of manufacturing amorphous metallic foams.